This invention relates generally to position encoders, and more particularly to a system and method for calibrating a delay between a position request and an acquisition time in an inductive position encoder.
Various position encoders for sensing linear, rotary or angular movement are currently available. These encoders are generally based on either inductive transducers, capacitive transducers, optical systems, or magnetic scales. In general, an encoder may comprise a transducer with a readhead and a scale. The readhead may comprise a transducer element and some transducer electronics. The transducer outputs signals which vary as a function of the position of the readhead relative to the scale along a measuring axis. The transducer electronics outputs the signals to a signal processor or processes the signals internally before outputting modified signals indicative of the position of the readhead relative to the scale. It is also common for an encoder system to include an interface electronics separate from the readhead, and to interpolate or otherwise processes the transducer signals in the interface electronics before outputting modified signals indicative of the position of the readhead relative to the scale to an external host.
Some position encoder systems communicate with an external host using a request and response process. This process may include three steps: a host computer sends a request for a position measurement; the encoder samples the output of the position transducer; and the encoder responds by transmitting position information. The delay between when the host computer sends the request for the position measurement and when the encoder samples the position transducer is called the sample delay. If the sample delay is significant, then in many applications it is important that the delay be known and constant. Knowing the amount of the delay is particularly important in certain motion control systems which use the delay information to tune the performance of their position control loop and/or estimate position corrections using velocity information. In other cases, it is simply important that the sample delay conform to the interface constraints of an existing motion controller or other host system.
One prior art patent which describes an encoder unit which accounts for processing delays is U.S. Pat. No. 5,721,546. The ""546 patent teaches an encoder in which a delay time of data which is caused by analog to digital conversion and arithmetic processing time is compensated to eliminate motion-related errors to prevent deterioration of control performance. The device operates by predicting a position change that occurs during a delay time in accordance with angular data obtained from current and previous sampling cycles. The error related to delay time is then compensated for by adding the predicted position change to the current sampling data. However, the relatively complex processing in the encoder and the data interface between the encoder and host taught in the ""546 patent is not desirable in a number of applications. Furthermore, the method of the ""546 patent only compensates each encoder""s sample delay for predicted position changes based on angular data from current and previous sampling cycles, and thus does not address other issues associated with host interface timing constraints, signal processing complexity and the like. For example, the method of the ""546 does not make a sample delay time more predictable, or conveniently and economically establish a sample delay time that is to a high degree the same for a plurality of interchangeable encoders. Alternatively, it is known to make a sample delay time more predictable and establish a sample delay time that is to a high degree the same for a plurality of interchangeable encoders by including a relatively accurate high speed clock in each encoder readhead. However, there are number of applications for encoders in which the readhead size and power consumption must be reduced to a practical minimum. In such applications, significant readhead signal processing and/or including an accurate high speed clock in the readhead are effectively prohibited.
The present invention is directed to providing a method and apparatus that overcome the foregoing problems and disadvantages. More specifically, the present invention is directed to a method and apparatus for providing a calibrated delay between a position request to a position encoder and a sample acquisition time in the encoder readhead.
A system and method is provided for calibrating a delay time between a position request and an acquisition time in a position encoder system. The position encoder system includes a scale, a readhead with a transducer and transducer electronics, and interface electronics which control the readhead and obtain measurements from the readhead. The interface electronics are accessible from a host computer. The transducer of the readhead may be an inductive transducer. When an inductive transducer is used, the clock frequency used for sample acquisition may be somewhat slow and variable since it may be partially determined by the resonance between the inductance of the transducer pattern and the multiple tuning capacitors. The clock included in the interface electronics is more accurate than the clock of the readhead, and is generally used for the timing calibration procedures.
In accordance with one aspect of the invention, the sample delay time of the position encoder system is calibrated by measuring an initial sample delay time and then comparing it to a specification delay time. The difference between the initial delay time and the specification delay time is designated as the delay time calibration. The delay time calibration is saved to memory and then included in the measurement process so as to make the calibrated sample delay time the same as the specification delay time and consistent between different position encoders.
In accordance with another aspect of the invention, the sample delay time of the position encoder may be measured a number of times so that a more accurate delay time calibration for the position encoder can be determined. In one embodiment, the measurement of multiple sample delay times may reduce the uncertainty of the measurement by one-half or more. The uncertainty of the measurement is sometimes referred to as jitter. In one specific implementation of the position encoder system, the amount attributable to jitter may be reduced from plus or minus 100 nanoseconds to plus or minus 50 nanoseconds.
In accordance with yet another aspect of the invention, in one embodiment the position encoder utilizes symmetric sampling. During the symmetric sampling process, the analog transducer output signals are read and integrated over two time intervals. The sample delay time may be defined as the median time between the two integration intervals.
In accordance with still another aspect of the invention, the number of communication lines required for the position encoder readhead is kept to a minimum. This is partially accomplished by making the internal timing signal of the readhead available at the normal output of the readhead when a specific calibration process is being performed. This eliminates the need for an additional communication line for this purpose.
It will be appreciated that the disclosed systems and methods for the calibration of the sample delay time are advantageous in that they allow the calibrated sample delay to be made the same as the specification delay time and consistent between different encoder units. The systems and methods have particular utility in combination with small sized encoder readhead heads which exclude complex circuits and accurate high speed clocks.